Current processes used to produce powder metal products typically employ conventional fluid atomization techniques to produce alloy powders. For example, conventional fluid atomization technology is used to produce alloy powders for the production of common pressed and sintered articles. Alloy powders also are used in more sophisticated settings, such as in the fabrication of materials from which critical aerospace components are fabricated.
In one conventional fluid atomization process, high pressure gas is impinged on a molten metal or alloy stream and physically breaks the stream up into small particles of fully or partially molten material. As these molten particles dissipate heat, they freeze, and they are collected as a solid powder. In certain critical applications, such as in the fabrication of certain aerospace components, batches of powder atomized from several small atomization runs are blended, and then the blend is sieved to small size (for example, −325 mesh), containerized in a metallic can, and consolidated into a suitable solid article (preform) by extruding or otherwise compacting the can and its powdered contents. The consolidated article can then be further processed into the desired shape and character by machining and other conventional techniques. Advantages of this process include the cleanliness, controlled and uniform composition, and relatively small grain size of the consolidated article, which may be critical to the performance of a component fabricated from the article.
The conventional process, combining steps of melting, atomization, blending, sieving, containerizing, and consolidating, suffers from several drawbacks. For example, the atomized powder from several small melts is used to form the blended powder. This is done since a melt must be poured through a relatively small orifice during powder formation, and the pour rate is significantly less than is used in casting or conventional melting. Thus, prior to being atomized, the alloy must remain molten for an extended period, which can result in deterioration of the alloy's chemical composition, through elemental volatilization and reactions with the ceramic liner of the melting vessel. Several small melts are atomized so as to minimize compositional deterioration of any one melt. Accordingly, the powder forming process is typically time-consuming and capital intensive. Also, the melts typically are produced in conventional ceramic-lined furnaces and, hence, the resultant powders are often contaminated with oxides. Once the powders are formed, they are then handled in several steps, each of which presents the possibility, and likelihood, of additional contamination. Also, because the process includes several steps, it is typically costly.
Various techniques have been developed to specifically address distinct steps in the process of forming consolidated articles from a melt using powder atomization. Several well known melting techniques have been developed that employ a vacuum environment and do not use a ceramic-lined furnace. These techniques result in significantly less oxide contamination in the melt relative to forming the melt in a conventional ceramic-lined furnace. For example, electron beam (EB) melting technology is now widely known and broadly discussed in the technical and patent literature. Another example is the vacuum double-electrode remelting (VADER) process, which is known in the art and described in, for example, U.S. Pat. No. 4,261,412. Other known techniques of forming molten alloy streams in ceramic-less melting devices are disclosed in, for example, U.S. Pat. Nos. 5,325,906 and 5,348,566. The '906 patent discloses a melting apparatus combining an electroslag remelting (ESR) device coupled to a cold induction guide (CIG). In one embodiment described in the '906 patent, a stream of molten refined material is produced by melting a consumable electrode in an ESR device. The molten stream passes, protected from the environment through a closely coupled CIG, downstream to a spray forming device. The '566 patent similarly discloses an apparatus combining an ESR device closely coupled to a CIG, but further discloses techniques for controlling the flow of molten material through the CIG. The techniques include, for example, controlling the rate of induction heat supplied to the alloy within the CIG, and controlling the rate of heat removal from the molten material within the CIG, through the cold finger apparatus itself and through an adjacent gas cooling means.
In conventional fluid impingement atomization techniques, either a gas or a liquid is impinged on a stream of a molten material. Impingement using liquid or certain gases introduces contaminants into the atomized material. Also, given that fluid impingement does not occur in a vacuum environment, even impingement techniques using inert gases can introduce significant impurities into the atomized material. To address this, certain non-fluid impingement atomization techniques that may be conducted in a vacuum environment have been developed. These techniques include atomization processes described in U.S. Pat. No. 6,772,961 B2, entitled “Methods and Apparatus for Spray Forming, Atomization and Heat Transfer” (“the '961 patent”), wherein molten alloy droplets or a molten alloy stream produced by a melting means coupled with a controlled dispensing means are rapidly electrostatically charged by applying a high voltage to the droplets at a high rise rate. The electrostatic forces set up within the charged droplets cause the droplets to break up or atomize into smaller secondary particles. In one technique described in the '961 patent, primary molten droplets produced by the nozzle of a dispensing means are treated by an electric field from a ring-shaped electrode adjacent to and downstream of the nozzle. Electrostatic forces developed within the primary droplets exceed the surface tension forces of the particles and result in formation of smaller secondary particles. Additional ring-shaped field-generating electrodes may be provided downstream to treat the secondary particles in the same way, producing yet smaller molten particles. The entire disclosure of the '961 patent is hereby incorporated herein by reference.
Electron beam atomization is another non-fluid impingement technique for atomizing molten material, and is conducted in a vacuum. In general, the technique involves using an electron beam to inject a charge into a region of a molten alloy stream and/or a series of molten alloy droplets. Once the region or droplet accumulates sufficient charge the Rayleigh limit, the region or droplet becomes unstable and is disrupted into fine particles (i.e., atomizes). The electron beam atomization technique is described generally in the '961 patent, and is further described below.
The '961 patent also discloses techniques using electrostatic and/or electromagnetic fields to control the acceleration, speed, and/or direction of molten alloy particles formed by atomization in the process of producing spray formed preforms or powders. As described in the '961 patent, such techniques provide substantial downstream control of atomized material and can reduce overspray and other material wastage, improve quality, enhance the density of solid preforms made by spray forming techniques, and improve powder quality and yield when atomizing material to a powder form.
In connection with collecting atomized powders, the method of letting atomized powders settle on the bottom of an atomization chamber is known and has been routinely used commercially in the manufacture of alloy powders. Also, methods of collecting atomized materials as unitary preforms, such as, for example, spray forming and nucleated casting, are well known and have been described in numerous articles and patents. With respect to nucleated casting, specific reference is drawn to U.S. Pat. Nos. 5,381,847, 6,264,717, and 6,496,529 B1. In general, nucleated casting involves atomizing a molten alloy stream and then directing the resultant particles into a casting mold having a desired shape. The droplets coalesce and solidify as a unitary article in the shape of the mold, and the casting may be further processed into a desired component. Spray forming involves directing atomized molten material onto a surface of, for example, a platen or a cylinder to form a free-standing preform. Characteristically, the typical solids fraction of the atomized particles differs between spray forming and nucleated casting since, for example, a less fluid and mobile particle is necessary in the mold-less spray forming process.
As noted above, many of the known processes for melting, atomizing and forming alloys to produce powders and solid preforms have deficiencies. Such deficiencies include, for example, the existence of oxides and other contaminants in the final product, yield losses due to overspray, and inherent size limitations. Accordingly, there is a need for improved methods and apparatus for melting and atomizing alloys and forming powders and solid preforms from the atomized materials.